The present invention relates to a Drop-On-Demand ink jet recording head for use in an ink-jet recording apparatus, and particularly to an ink jet recording head by which an ink material is ejected in response to an image signal and then deposited onto a recording medium such as plain paper to reproduce an image.
There have been proposed a variety of Drop-On-Demand ink-jet recording head. The recording head includes a pressurizing member, such as a heat member or piezoelectric member, which pressurizes an ink material in an ink cavity in response to an image signal so as to eject ink droplets through a nozzle into the air. The ejected ink droplets are deposited on a recording medium such as plain paper to form an image consisting of a number of dots.
One known method for reproducing a continuous tone image by the use of such ink-jet recording head, is to change the size of ink droplets ejected through the nozzle so as to change the size of dots deposited on the recording medium. For this purpose, there has been proposed an ink-jet recording head which includes large nozzles for ejecting large ink droplets and small nozzles for ejecting small ink droplets. Specifically, U.S. Pat. No. 5,208,605 discloses a printer head in which a large-nozzle array and a small-nozzle array are arranged in parallel. Also, Japanese Patent Laid-Open Patent Publication 6-238092 discloses an ink jet printer head in which large nozzles and small nozzles are arranged alternately in a row.
Technically, however, it is difficult to form a multiplicity of large and small nozzles of different sizes in one head with a great precision such as on the order of a micron. Further, there is a limitation in the adjustable range of nozzle diameter, for example, due to the fact that reducing the nozzle diameter too much would make it more likely that clogging takes place in the small nozzle. As a result, it would be difficult to realize sufficiently wide, smooth gradation by varying the dot diameter only with different nozzle diameters.
Among the several objects and features of the invention may be noted the provision of an ink-jet recording head for use in an ink-jet recording apparatus, and the provision of an ink-jet recording head capable of ejecting ink droplets of different sizes.
An ink-jet recording head of the invention comprises first and second head portions. Each head portion has a member which includes an ink cavity for receiving a ink material and a nozzle fluidly communicated with the ink cavity for ejecting the ink material. The head portion has a piezoelectric actuator. The actuator deforms to pressurize the ink material to eject the same through the nozzle into the air, when a voltage is applied thereto. The ink-jet recording head is characterized in that the sizes of both of the ink cavity and the associated piezoelectric actuator in the first head portion are different from those in the second head portion.
According to the ink jet recording head of the invention, since the ink cavity and piezoelectric actuator in the first head portion have different sizes from those in the second head portion, the pressure that acts on the ink material in the first head portion is different from that in the second head portion even when the same voltage is applied to piezoelectric actuators in the first and second head portions. Responsive to the difference in the pressure, the size of ejected ink droplet and the size of deposited ink dot by the first head portion will be different from those by the second head portion. Thus, according to the invention, a wider, smooth tone gradation can be realized.
The nozzle in the first head portion may be the same size as that of the second head portion. In this case, the nozzle is unlikely to suffer clogging, and it can be formed with a great precision.
Preferably, the ink-jet head comprises a controller which controls the voltage to be applied to piezoelectric actuator for changing the size of ink droplet to be ejected through the nozzle of the head portions. With this ink-jet recording head, an energy for pressurizing the ink material in the ink cavity by piezoelectric actuator will be changed depending upon the voltage to be applied to piezoelectric actuator, thereby changing the size of ink droplet to be ejected from the nozzle of each head portion and then the size of resultant ink dot on the recording medium in many steps. This results in a wider, smooth and continuous tone gradation.
Additionally, even when the voltage applied to piezoelectric actuators in the first and second head portions are changed in the same manner, since the sizes of the ink cavity and piezoelectric actuator in the first head portion are different from those in the second head portion, the sizes of the ink droplets ejected by the first head portion change in a range different from that by the second head portion. Therefore, the ink-jet recording head can be designed so that the first head portion ejects relatively small ink droplets while the second head ejects relatively large ink droplets and thereby causes the size of ink droplets to vary widely without increasing the load on the drivers of the head portions. Further, this allows the drivers to be simplified in construction and lowered in price.